1. Field of the Invention
The present invention relates to an optical film having an anti-static function and an anti-reflection function, and a method for producing the same.
2. Description of the Related Art
High-definition large-screen displays, typified by plasma display panels (PDPs) and the like, have been rapidly developed. It is necessary to dispose an anti-reflection layer having an anti-reflection function on the surface of displays, in order to prevent outside light from being reflected on the screen thereof. Furthermore, displays are required to have an anti-static function in order to prevent dust, filth, or the like from being attached to the screen surface due to static electricity. Thus, in conventional displays, an optical film having an anti-reflection function and an anti-static function is disposed on the front face of the screen.
This conventional optical film is formed by disposing an anti-reflection layer on a transparent base, wherein the anti-reflection layer often has a three-layer structure in which a hard coating layer, a high refractive index layer with a thickness of 1 μm or less, and a low refractive index layer are laminated in this order from the side of the transparent base. Furthermore, an anti-static function usually is provided by adding an anti-static material to the high refractive index layer (see JP 2002-200690A, for example).
As the anti-static material, a conductive metal oxide often is used in conventional examples. However, many conductive metal oxides that absorb light with a specific wavelength are colored, and thus if the thickness of an anti-static layer to which these conductive metal oxides have been added is more than 1 μm, or if the amount thereof added is excessive, then there is a problem in that the total light transmission of the optical film is reduced. Furthermore, there also is a problem in that light is prevented from traveling by conductive metal oxide particles, and thus the internal scattering of light increases the haze. In conventional examples, an anti-static material is added to a high refractive index layer with a thickness of 1 μm or less as described above, and thus the total light transmission is reduced to a lesser extent and the haze increases to a lesser extent.
On the other hand, in recent years, it has been proposed that an anti-reflection layer have a two-layer structure comprising a hard coating layer and a low refractive index layer, without a high refractive index layer, in order to reduce the number of production processes (see JP 2000-233467A, for example).
However, according to JP 2000-233467A, a considerable amount of fine particles of a conductive metal oxide are added as an anti-static material to a conductive transparent layer with a thickness of several μm that is disposed under the low refractive index layer, and thus the total light transmission is less than 91% even at the maximum, and the haze is 1.0% even at the minimum. In other words, the transmittance is not sufficient.
On the other hand, it also has been proposed to reduce the amount of the conductive metal oxide added inside the hard coating layer (see JP 2003-306561A, for example).
However, JP 2003-306561A does not describe an anti-reflection layer at all, and the total light transmission is less than 91% even at the maximum, and the haze is 0.9% even at the minimum, that is, the transmittance is not sufficient as well as in JP 2000-233467A. Furthermore, JP 2003-306561A discloses a method for producing an anti-static hard coating film, comprising a step of performing humidification in an atmosphere with a relative humidity of 35 to 70%, but the relative humidity changes depending on the temperature, and thus the production conditions cannot be constant, so that there is a problem in that the characteristics of produced films are not constant.